Tryptophan, .alpha.-amino-.beta.-indolepropionic acid is an amino acid originally isolated from enzymatic digests of proteins. Various tryptophan compounds can be obtained by extraction, fermentation or chemical synthesis. Extraction is a difficult process because tryptophan is easily destroyed during acid hydrolysis of proteins, and racemization occurs on alkaline hydrolysis. Fermentative methods include the production of tryptophan from anthranilic acid or indoles. The chemical synthetic methods are generally of two types: (1) syntheses in which an alanyl residue is added to a preformed indole and (2) syntheses in which the indole portion of the molecule is formed late in the sequence. The latter chemical syntheses are preferred because the preformed indoles of the former chemical syntheses are costly raw materials. Furthermore, the latter chemical syntheses involve a minimum of conversions during which the chemically sensitive indole nucleus might be destroyed. However, the latter chemical routes are not ideal because the required indole formation usually involves cyclization into an aromatic ring which leads to undesirable mixtures of isomers. The formation of isomers is particularly acute in the case of 4- and 6-substituted tryptophans.
The instant invention provides a means for the obtention, in high yield, of racemic and optically pure tryptophans, the D-enantiomers of which have potential value as non-nutritive sweeters.
Numerous substances have been proposed and/or used as non-nutritive sweetening agents, affording the consumer ingesting the same a sense of sweetness at least desirably comparable to that obtained with natural sugar, but without caloric effect. Such substances are necessary for some persons in order to limit intake of the natural sugars and thereby to control various health conditions, including diabetes. Many of these substances, however, have severe disadvantages. The most frequently encountered disadvantages are a bitter aftertaste and toxic side effects at rates not substantially different from those at which the sweetening effect is obtained. Only two classes of non-nutritive sweetening substances are used to any extent: saccharin-type compounds and cyclamate-type compounds. Both classes have the typical disadvantage of a bitter aftertaste; and in addition, cyclamate-type compounds have only limited activity.
Among the various categories of chemicals which have been evaluated for sweetening effect are the amino acids. A recent publication, Vuataz et al., Experientia, Vol. XXI, pages 692-694, inclusive (1965), reports the evaluation of a number of amino acids, the enantiomorphs being evaluated separately where available. The report shows that while a number of amino acids are sweet in the D form, this is not an absolute correlation. Furthermore, despite the sweet taste of the D-enantiomorph of a given amino acid, the L-enantiomorph of the same amino acid may be bitter. Resolution of the two enantiomorphs of a given amino acid is often difficult. For these various reasons, despite the contribution of Vuataz et al., no amino acid is being used in the sweetening art.
Moreover, throughout the sweetening art, it is well known that there is no correlation between structure and activity, as even relatively small changes in chemical structure often destroy activity.
In recent years the commercially-available synthetic sweetners, saccharin and cyclamate, have encountered some difficulties in toxicological studies. Aftertaste problems have also plagued these products. It has been evident, therefore, that a need exists for new sweetening agents. This need has been met, in part, by the new sweetener 1-aspartyl-1-phenylalanine methyl ester (Aspartame.RTM.), but this dipeptide ester has shown instability under some conditions.
It has recently been discovered that certain substituted tryptophan compounds in their D form exhibit a sweet taste of a marked degree. See U.S. Pat. No. 3,899,592 to Suarez et al. and U.S. patent application Ser. No. 698,573 to Batcho et al., filed June 22, 1976. Substituted D tryptophans thus are important as artifical sweetners.
Additionally, because tryptophan in the L form is an essential amino acid, it has importance as a food supplement.
According to the present invention, tryptophans (unsubstituted or substituted) of the D or L forms are prepared by novel processes via novel intermediates which avoid the above described difficulties of the prior art processes.